Specify Swift names for GIDClaim methods

[w-goog]

The Obj-C method - (instancetype)AMRClaim; gets rewritten to GIDClaim.amr() by Swift's Obj-C importer. This is ~fine, but a) not discoverable by reading the file and b) could plausibly change if the Obj-C importer changes its behaviour. That is not very nice, so, this PR pins the names.

(Does this seem like a silly thing to spend time on? Yes. Did it already bite once and require a developer docs update? Also yes)

[w-goog]

Here is the pre- post- diff of the API, to ensure that it is identical:

$ xcrun swift-api-digester -dump-sdk -module GoogleSignIn -I before -I $FETCHER_HEADERS -o before.json …
$ xcrun swift-api-digester -dump-sdk -module GoogleSignIn -I after -I $FETCHER_HEADERS -o after.json …

PRE (origin/main, 0 annotations):
authTimeClaim -> authTime()
essentialAuthTimeClaim -> essentialAuthTime()
AMRClaim -> amr()
essentialAMRClaim -> essentialAMR()

POST (branch, 4 annotations):
authTimeClaim -> authTime()
essentialAuthTimeClaim -> essentialAuthTime()
AMRClaim -> amr()
essentialAMRClaim -> essentialAMR()